2. Enabling Network Service: By default, the ability to interact with network services is disabled in Oracle Database 11g or later. Therefore, you must use the BMS_NETWORK_ACL_ADMIN package to grant connect privileges to any host for the database user. In this tutorial i will create network service for APEX 4.x power user that means APEX_040200 (Power user for APEX 5 is APEX_050000):

DECLARE
ACL_PATH VARCHAR2(4000);
BEGIN
-- Look for the ACL currently assigned to '*' and give APEX_040200
-- the "connect" privilege if APEX_040200
-- does not have the privilege yet.
SELECT ACL INTO ACL_PATH
FROM DBA_NETWORK_ACLS
WHERE HOST = '*' AND LOWER_PORT IS NULL AND UPPER_PORT IS NULL;
IF DBMS_NETWORK_ACL_ADMIN.CHECK_PRIVILEGE (ACL_PATH,'APEX_040200','connect')
IS NULL THEN
DBMS_NETWORK_ACL_ADMIN.ADD_PRIVILEGE(ACL_PATH,'APEX_040200', TRUE, 'connect');
END IF;
EXCEPTION
-- When no ACL has been assigned to '*'.
WHEN NO_DATA_FOUND THEN
DBMS_NETWORK_ACL_ADMIN.CREATE_ACL('power_users.xml', 'ACL that lets power users to connect to everywhere', 'APEX_040200', TRUE, 'connect');
DBMS_NETWORK_ACL_ADMIN.ASSIGN_ACL('power_users.xml','*');
END;